1. Field of the Invention
This invention provides for improved sterol nucleators of ice crystals. The improvement is the use of water-stable crystalline forms of the sterols. These forms advantageously provide predictable nucleation temperatures which are stable for long periods of time, especially where the water is repeatedly frozen and thawed. In commercial ice making systems, the selection of stable forms of sterols having high nucleation temperatures provide great savings in energy and construction costs.
By way of background, pure water will supercool before forming ice. Depending upon the purity, water may be cooled to -10.degree. to as low as -40.degree. C. before crystallizing into ice. Under optimum conditions, the transition to ice can be quite rapid with the heat of crystallization raising the temperature of the water from its supercooled state to the 0.degree. C. typical of ice. This crystallization temperature can be raised by the addition of nucleators, such as mineral particles, proteins, terpenes or sterol crystals.
Although sterols are known to be nucleators of ice, their uses have been mostly limited to situations where the water is not repeatedly frozen and thawed. Moreover, the sterols of prior art were mostly dehydrated and gave unreliable nucleation temperatures. Prior art taught that nucleation performance could be improved by procedures known to promote formation of anhydrous crystals such as solidification from the melt or crystallization from a dry, non-polar solvent such as benzene.
My investigations confirm that prior art sterols have significantly reduced nucleation ability after extended water contact. This renders them relatively costly for larger commercial systems where construction and operating costs are important. In contrast and surprisingly, intentionally hydrated sterols have been discovered to be vastly superior as ice nucleators and offer instances of stable ice nucleating performance at above -2.degree. C. where very few nucleators of any type (e.g., metaldehyde, fluorophlogopite mica) have ever been reported. This invention identifies examples of such sterols and discloses a simple means to readily identify other such sterols.
The concept of a "threshold or onset nucleation temperature" as used in prior art is inappropriate for the invention disclosed here. Here it should be appreciated that the probability of a unit freezing in a multi-unit ice making system is statistically distributed around the nucleation temperature. The width of such a distribution with temperature may be unique to each nucleator. For this reason and as further explained below, it is necessary to chill the sterol suspensions a degree or two below the nucleation temperature. This ensures that sufficient units actually crystallize into ice (e.g., 90% crystallize during a chilling cycle). The optimum chilling temperature is readily obtained by routine experimentations and application of basic statistical methods.
Thus, it should be understood that when refrigeration devices lower the ambient temperature of individual units or vessels containing the solutions of this invention to the nucleating temperature, approximately 50% of the units will freeze. For this reason the refrigeration device must chill the vessels at least one degree below the nucleation temperature to be of practical use.
One of the problems this invention solves is that nucleating temperatures of greater than -3.degree. C. permit the adaption of existing air conditioning systems without replacing the existing compressors supplying a building's air conditioning or refrigeration. The existing standards for many air conditioners of commercial buildings do not permit the refrigeration of a brine heat transfer liquid to -9.degree. C. required to freeze the sterol-containing building chilling systems of the prior art.
Moreover, use of such low temperatures requires additional amounts of additives to the heat transfer liquid (more salt or ethylene glycol). This decreases the efficiency of the liquids and increases their cost. It is estimated that running a heat transfer liquid at -7.degree. to -9.degree. C. rather than 2.degree. to -3.degree. C. requires 16% more energy and 27% greater cooling capacity for the refrigeration devices (e.g., centrifugal compressors). Thus the prior art disclosing cholesterol with a nucleation temperature of -7.2.degree. C. is not useful without substantial modifications to many existing systems.
2. Information disclosure
Dry cholesterol has been used as nucleators of ice crystals in chilled water systems. U.S. Pat. No. 4,928,493. It was subsequently discovered that in multiple tube quiescent tests, freshly crystallized cholesterol nucleated half the water samples within one hour at -1.9.degree. C. When the cholesterol is terminally hydrated after a few freezing cycles or by standing at ambient temperature, it degrades into a low temperature nucleator, active between about -6.degree. C. and -8.degree. C. More precisely, samples initially showing 50% freezing at -1.9.degree. C. later did not freeze at 3.8.degree. C., 17% froze at -5.8.degree. C., and 67% froze at -7.8.degree. C. in successive one hour quiescent tests.
By plotting the 0.17 and 0.78 probabilities of freezing versus tests temperature on probability paper, linear interpolation locates the 0.50 probability at -7.2.degree. C., the nucleation temperature of terminally hydrated cholesterol. When the same line is extrapolated to the 0.95 probability level, the temperature is -9.5.degree. C. It should be appreciated that in an ice making system with thousands of individual freezing vessels, these data indicate that a maximum brine temperature of below -9.5.degree. C. is required for prompt nucleation and utilization of 95% of the freezing vessels. If the brine temperature is higher, there will be an even greater fraction of inactive vessels in each cycle. The number of active vessels must be kept constant to maintain a constant thermal capacity, so a greater number of vessels must be employed. This illustrates why cholesterol nucleator necessitates excessive cooling of the brine, excessive installation size or both. Terminally hydrated cholesterol has utility in providing reproducible, but not efficient operation.
The nucleation of ice from vapor is reviewed in H. R. Byers, Nucleation in the Atmosphere, Indust. and Eng. Chem. 57(11), 32-40, 1965 and in A. C. Montefinale et al., Recent Advances in the Chemistry and Properties of Atmospheric Nucleants: a Review, Pure and Appl. Geophys. 91(8), 171-210, 1971. Others have reported that dry, non-hydrated sterols can act as nucleators of ice from water vapor. R. B. Head, Steroids as Ice Nucleators, Nature, 191, 1058-1059, 1961; R. B. Head, Ice Nucleation by Some Cyclic Compounds, J. Phys. Chem. Solids, 23, 1371-1378, 1962. The nucleation temperatures of several sterols were reported by N. Fukuta and B. J. Mason, Epitaxial Growth of Ice on Organic Crystals, J. Phys. Chem. Solids 24, 715-718, 1963 and in N. Fukuta, Experimental Studied of Organic Ice Nuclei, J. Atm. Sci. 23, 191-196, 1966. However, the nucleation temperatures represent estimates from quick tests using water vapor or soap films. The ability of sterols to nucleate ice crystals repeatedly at a stable temperature from the liquid phase has not been evaluated.
The sterols have been studied as to their crystalline structure. J. D. Bernal, et al., X-Ray Crystallography and the Chemistry of the Steroids. Part I, Phil. Trans. Roy. Soc. London, No 802, Vol 239A, 135-182, 1940; and, W. L. Duax, et. al. Conformational Analysis of Sterols: Comparison of X-Ray Crystallographic Observations with Data from Other Sources, Lipids 15(9), 783-792, 1980. Different crystalline phases of cholesterol have been reported. C. R. Loomis, et. al. The Phase behavior of hydrated Cholesterol, J. Lipid Res. 20, 525-535, 1979.
Very high temperature nucleators are rare. An example can be found in U.S. Pat. No. 3,858,805.